Because natural gas is colorless and odorless, many techniques to odorize, or inject a liquid perfume into, a natural gas supply have been developed in an effort to increase the safety of this valuable energy source for the millions of consumers who use it. For every natural gas pipeline, a precise volume of odorant must be injected into the pipeline so that gas leaks are detectable. The volume of odorant required to properly odorize a pipeline depends on the flow rate and composition of the natural gas within the pipeline.
Proper odorization of natural gas is equally important in both high and low volume applications, but the present invention is particularly beneficial for proper odorization in small diameter pipeline, low volume applications having flow rates ranging from 0.5 mcf/hr to 2 mmcf/hr. In the natural gas distribution industry, a large diameter pipeline typically delivers natural gas from the field to a local distribution center. These large diameter pipelines generally have flow rates ranging from 1 mmcf/hr to 50 mmcf/hr. The local distribution center removes gas from the large diameter pipeline and then delivers gas to the homes and businesses located in a given community. The local distribution center removes gas from the large pipeline at multiple locations, and the center must odorize the gas removed at each location. Each one of these locations typically uses a variety of small diameter pipelines, depending on the size of the community being served and thus the volume of gas required. Small diameter pipelines are also utilized in field applications, and therefore the present invention is beneficial for use in this environment as well.
Several static techniques, that is, techniques which generally utilize no moving parts, have been developed for odorization in low volume applications. One such technique is commonly referred to as a wick odorizer. In a wick odorizer, an odorant reservoir is directly connected to a small diameter natural gas pipeline. The reservoir is partially filled with liquid odorant, and a wick is suspended with one end in contact with the odorant and the other end extending into the pipeline. The wick draws odorant from its reservoir end into the pipeline via capillary action, and the liquid odorant evaporates into the flowing gas of the pipeline from the pipeline end of the wick.
Another example of an existing static technique is a bypass odorizer. In a bypass odorizer, a constriction is formed in a small diameter pipeline, and a fluid bypass conduit is routed to exit the pipeline on the upstream side of the constriction and re-enter the pipeline on the downstream side of the constriction. An odorant reservoir that is partially filled with liquid odorant is connected to the bypass conduit between the point where the conduit exits the pipeline and the point where the conduit re-enters the pipeline. The constriction creates a pressure drop in the pipeline that causes some of the gas within the pipeline to flow through the bypass conduit and over the liquid odorant in the reservoir. The liquid odorant evaporates into the natural gas flowing through the bypass conduit, and the odorized natural gas then re-enters the pipeline. The bypass conduit can be equipped with valves to adjust the volume of odorant provided to the pipeline in response to downstream monitoring of odorant levels in the pipeline.
Unfortunately, these static techniques exhibit several problems. First, the volume of odorant injected into a pipeline is imprecise and is often unpredictable. Second, in a bypass odorizer, particulates fall out of the natural gas flowing above the liquid odorant in the reservoir and coat the surface of the liquid odorant in the reservoir, thus decreasing the evaporation of the odorant into the gas. Third, if an accident occurs, a natural gas distributor must be able to prove proper odorization at the exact time and location of the accident, and such proof is particularly difficult given the unpredictability of these static methods.
In large diameter pipelines, large displacement liquid pumps have been utilized to inject precise volumes of liquid odorant into such pipelines with more predictable results. Such large displacement pumps typically inject 0.2 cc to 6 cc of odorant per stroke of the pump. Such pumps are typically operated by a control system which monitors the flow rate in the pipeline and determines a corresponding stroke rate for the pump necessary to inject the proper amount of odorant into the pipeline. U.S. patent application Ser. No. 08/083,135, now issued as U.S. Pat. No. 5,406,970 on Apr. 18, 1995, and which is commonly assigned with the present invention and discloses an example of such a control system, is incorporated herein by reference.
Existing liquid odorant injection pumps, particularly such pumps having a small displacement in the range of 0.02 cc to 0.1 cc per stroke, suffer from an additional problem. For a variety of reasons which are later discussed in more detail, the odorant supplied to odorant injection pumps is often a liquid containing dissolved gas. This dissolved gas often prohibits the pumping of a precise volume of fluid per stroke, as is explained below.
Since gas is compressible and liquid is generally incompressible, each stroke of a pump compresses any dissolved gas before it is displaced. Whether the dissolved gas is displaced, instead of merely being compressed, depends on the volumetric efficiency of a given pump. For the purposes of this invention, volumetric efficiency is defined as the volume of fluid displaced from a pump chamber for each pump stroke divided by the total volume of the pump chamber at the full upstroke position of the pump. If a pump has a high enough volumetric efficiency, it will displace most, if not all, of the dissolved gas for each pump stroke.
Existing liquid odorant injection pumps do not have a high enough volumetric efficiency to displace all or substantially all of the dissolved gas within their pump chambers for each pump stroke. Therefore, the presence of dissolved gas prevents existing liquid odorant injection pumps from reliably displacing a precise volume of odorant per stroke. In addition, if this non-displaced gas accumulates during operation, the pump can become "vapor locked," meaning the pump is generally compressing gas instead of displacing liquid. Large displacement pumps often eventually work through a vapor lock condition by progressively displacing the accumulated non-displaced gas. However, existing small displacement pumps, such as those displacing 0.02 cc to 0.1 cc per stroke, exhibit significantly more vapor locking problems.
Pure gas pumps having a displacement of 0.02 cc to 0.1 cc per stroke have been developed. However, such pure gas pumps cannot reliably pump a liquid containing dissolved gas. Although such gas pumps typically have a very high volumetric efficiency, the orifices, chambers, and seals of these pumps are designed to displace gas, which has a larger molecular spacing than liquid. If such a gas pump is utilized to pump a liquid containing dissolved gas, the pump seals and valves tend to fail after an unacceptably short time of service.
Therefore, a critical need exists for an odorant injection pump with a displacement in the range of 0.02 cc to 0.1 cc of odorant per stroke that is capable of reliably pumping a precise volume of liquid odorant containing dissolved gas. Such a fluid pump is necessary for the proper odorization of small diameter natural gas pipelines, and such a pump may prove beneficial in other applications which require a small, precision volume of fluid or chemical injection.
It is therefore an object of the present invention to provide a fluid pump for displacing a precise volume of a liquid containing dissolved gas.
It is a further object of the present invention to provide such a pump that displaces 0.02 cc to 0.1 cc for each pump stroke.
It is a further object of the present invention to provide such a pump in which the precise volume of fluid displaced can be adjusted to a selected amount in the range of 0.02 cc to 0.1 cc for each pump stroke.
It is a further object of the present invention to provide such a pump that includes instrumentation for indicating a decrease in the volumetric efficiency of the pump chamber due to a pump seal failure.
It is a further object of the present invention to provide such a pump that has modular components that are easily removable for service, testing, or replacement.
It is a further object of the present invention to provide such a pump for injecting a liquid odorant containing dissolved gas into a small diameter natural gas pipeline.
It is a further object of the present invention to provide such a pump for injecting a liquid odorant containing dissolved gas into a small diameter natural gas pipeline that prevents odorant leakage into the environment due to a pump seal failure.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having references to the following specification together with its drawings.